In most cases, a laminate for producing therefrom a printed wiring board used in the electronic industry is manufactured by impregnating a glass cloth, kraft paper, a glass nonwoven fabric or the like with a thermosetting resin such as phenolic resin or epoxy resin, setting the thermosetting resin semi-cured to produce a prepreg with the semi-cured resin attached thereto, and laminating a copper foil on one or each of both surfaces of the prepreg to obtain a copper-clad laminate.
Further, a multilayer printed wiring board having two inner-layer circuits and two outer-layer circuits is produced as illustrated below.
The one or two surface copper foils of the aforementioned copper-clad laminate are subjected to etching or the lie for formation of one or two inner-layer circuits to produce an inner-layer member comprising the inner-layer substrate (prepreg) and the one or two inner-layer circuits. Separately, the same prepreg as mentioned above has a copper foil attached to one or both surfaces thereof and is then subjected to etching or the like to produce an outer-layer member comprising the outer-layer substrate (prepreg) and outer-layer copper foil circuit. Two such outer-layer members sandwich said inner-layer member in therebetween, after which these three members are heated under a pressure to produce a multilayer printed wiring board having two inner-layer circuits and two outer-layer circuits.
The laminate produced by the aforementioned manufacturing method has practically satisfactory heat resistance, electrical properties and chemical resistance for use as a material or blank for a printed wiring board.
However, the use of the prepreg as described above raises problems, examples of which are an uneven surface called a "glass fiber texture" formed on the surface(s) of the laminate after pressing thereof, the lowering of insulation reliability called "migration", and a difficulty in perforating the laminate with a laser beam because of the inclusion in the prepregs, between the inner layer circuit and outer-layer circuit, of glass fibers which can not be decomposed even at a high temperature.
In attempts to overcome these problems, there have hitherto been proposed methods for producing a multilayer printed wiring board without use of any prepreg between inner-layer circuit and outer-layer circuit. "Printed Circuit World Convention VI, I9" describes a method for producing a multilayer printed wiring board with use of resin-coated copper foils respectively having a plurality of resin layers which are different in cured state and covered with a protective film. However, this method raises problems as to disadvantageously complicated production steps due to application of resin layers differing in cured state onto the surface of copper foils, troublesome peeling of the protective films and costly use of the protective film.
The inventors of the present invention have already proposed an adhesive for copper foils which is useful for laminating a copper foil on a substrate such as a metal plate or a plastics plate, and also proposed on adhesive-coated copper foil (Japanese Patent Application No. 243,430/94). The adhesive and the adhesive-coated copper foil disclosed herein are designed to adhere to a material under lower pressure and temperature than conventional ones. However, they are not intended to be used for press lamination or multilayer relamination because an insulation layer of variable thickness would result and there would be high resin flow causing excessive resin "bleed out". Another problem is a so-called "blocking phenomenon" that is adhesion between the resin surface of one copper foil and the glossy surface of another copper foil adjacent to said one copper foil when the resin-coated copper foils are stacked one upon another for storage.
Further, Japanese Patent Application No. 22,321/95 has proposed an adhesive composition to achieve the low resin flow even during the press lamination. The resin composition disclosed in the above Application comprises 20 to 50 wt/%, based on the total amount of the resin components, of a polyvinyl acetal resin thereby to control the resin flow during the course of press molding. The intention of this invention is to produce a printed wiring board while eliminating the conventional copper foil roughening step. However, there may be defects such as voids in the inner layer circuits after relamination. Furthermore, since a malamine resin is constituent of the adhesive it may be attached by etching solution during subsequent processing.
In Japanese Patent Laid-Open Gazette No. 106,752/95, the inventors of the present invention have pointed out or indicated problems which will be raised as to the use of a melamine resin as an adhesive for use in a printed wiring board. According to this specification the inclusion of melamine resin in the adhesive used to coat copper foil for printed wiring board applications results in chemical attack by copper chloride/hydrochloric acid etching solutions. This will raise a problem that copper ions remain on the surfaces of the substrate even if the etched copper foils are washed with water and these copper ions inhibit the cure of a solder resist ink applied on the surfaces of the substrate thereby lowering the adhesion of the ink to the substrate. Since the melamine resin is disclosed to be used as part of the resin composition in Japanese Patent Application No. 22,321/95 as well, it is difficult to obviate the foregoing problem.